Ostracods are small bivalved crustaceans that are common in non-marine waters. They secrete shells of low-Mg calcite, which are often well preserved in Quaternary sediments. Ostracods are sensitive to a range of ecological factors, of which habitat type and the chemical composition are potentially most valuable for palaeoenvironmental work. Furthermore, ostracod shells provide a source of carbonate for geochemical analysis, including trace elements and stable isotopes. Despite past under-representation of ostracods in palaeolimnology, recent advances in the understanding of their biology, taxonomy, ecology and shell chemistry have led to them becoming used almost routinely in environmental reconstruction alongside other indicators. This chapter reviews the palaeolimnological significance of ostracods by providing an outline of their biology, taxonomy and ecology, together with an account of the main palaeoenvironmental techniques and their application.